This invention relates generally to data processing, and more particularly to a method and system for sampling a signal using analog-to-digital converters.
Many different types of analog-to-digital converters are known. One type of analog-to-digital converter is a xe2x80x9cflashxe2x80x9d analog-to-digital converter. Flash converters use a parallel architecture to sample an analog input signal and generate multiple-bit digital outputs. Another type of analog-to-digital converter is the Delta-Sigma (xcex94xcexa3) converter. One-bit Delta-Sigma converters generate digital outputs using the analog input signal and feedback from the prior digital output. xe2x80x9cHigh-orderxe2x80x9d Delta-Sigma converters typically use many filters to generate a digital output, while xe2x80x9clow-orderxe2x80x9d Delta-Sigma converters typically use fewer filters. In this document, the terms xe2x80x9clow-orderxe2x80x9d and xe2x80x9chigh-orderxe2x80x9d are used to denote relative orders of the converters, and are not intended to correspond to any particular range of orders. Low-order Delta-Sigma converters are popular because of their simple design and their insensitivity to manufacturing tolerances.
Communication systems often use analog-to-digital converters to sample analog input signals. The analog signals may contain information that will be processed by the communication system. Typically, the analog-to-digital converter receives the analog signal, samples the analog signal at different times, and generates a digital representation of the analog signal at those times. Each digital representation forms a digital output signal that represents the analog input signal. A processor or other computing device in the communication system uses the digital output signal to approximate the analog signal.
Some applications in the communication system do not require a high level of precision in the sampling of the analog signal, while other applications require precision sampling. Lower-order Delta-Sigma converters have typically not been used in applications that require precision sampling for at least two reasons. First, Delta-Sigma converters typically suffer from the formation of xe2x80x9ctonesxe2x80x9d in the digital output signal. Digital and analog noise in the converter may not be completely random, and low-order Delta-Sigma converters generate one digital output using feedback from the prior digital output. Correlations in the noise may lead to the creation of periodic bit patterns in the feedback loop of the Delta-Sigma converter, which leads to the creation of a spike, or tone, in the time-domain Fourier spectrum of the digital output. The formation of tones typically cannot be avoided in low-order Delta-Sigma converters. These spikes appear to the communication system as part of the information communicated over the analog signal. Because of this, lower-order Delta-Sigma converters cannot typically be used in applications that require precision sampling of the input signal.
Lower-order Delta-Sigma converters have also typically not been used in applications that require precision sampling because of their lower signal-to-noise power ratio. The signal-to-noise power ratio represents the power of the useful information generated by a converter compared to the power of the noise or undesired signals generated by the converter. The order of the Delta-Sigma converters typically determines the degree of noise shaping, and thus noise reduction, at frequencies near that of the analog input signal. Lower-order Delta-Sigma converters typically have lower signal-to-noise power ratios than the higher-order converters. As a result, the higher-order converters typically provide greater precision than the lower-order converters.
Approaches to providing precision sampling typically involve using a flash converter or a higher-order Delta-Sigma converter. These types of converters have typically provided greater precision in the sampling of the analog signal. A problem with this approach is that typical flash converters and higher-order Delta-Sigma converters generally include more components than other types of converters. The use of more components increases the cost of the communication system and increases the complexity of manufacturing the converters.
Another problem with this approach is that prior attempts to eliminate tones in the communication system often reduce the effectiveness of the converter or add complexity to the converter. For example, eliminating tones in a higher-order Delta-Sigma converter typically requires the use of multiple feedback paths within the converter, which increases the complexity of the converter.
Tones in flash converters typically result from unavoidable errors that occur in the discretization of a continuous analog input signal into a limited number of possible digital output values. The minimum amplitude of tonal errors is typically set by the number of effective output bits provided by the converter. Eliminating tones in a flash converter typically requires the introduction of noise to the analog input signal. This is often referred to as xe2x80x9cditheringxe2x80x9d the analog signal. However, dithering reduces the signal-to-noise power ratio, and therefore the effectiveness, of the flash converter.
A further problem with this approach is that a higher-order Delta-Sigma converter may suffer from instability from variations in the manufacturing process. Higher-order Delta-Sigma converters are not as insensitive to manufacturing tolerances as the lower-order converters. As a result, the performance of higher-order converters may be strongly affected by the materials used in their construction and imperfections in the circuitry.
The present invention recognizes a need for an improved method and system for sampling a signal using analog-to-digital converters. The present invention reduces or eliminates at least some of the shortcomings of prior systems and methods.
In one embodiment of the invention, a system for sampling an input signal includes a plurality of analog-to-digital converters operable to convert the input signal into digital output signals. At least one of the analog-to-digital converters is also operable to receive a bias voltage different than a bias voltage received by at least one other analog-to-digital converter and to convert the input signal into the digital output signal using the bias voltage. The system also includes a digital accumulator coupled to the analog-to-digital converters. The digital accumulator is operable to receive the digital output signals from the analog-to-digital converters and to generate a net digital output signal comprising a sum of the digital output signals.
In one particular embodiment of the invention, the analog-to-digital converters comprise first-order or second-order Delta-Sigma analog-to-digital converters. In another particular embodiment of the invention, all of the analog-to-digital converters are operable to receive different bias voltages, such as between +5 mV and xe2x88x925 mV. In another embodiment of the invention, a method for sampling an input signal includes receiving the input signal, and converting the input signal into a plurality of digital output signals using a plurality of analog-to-digital converters. At least one of the analog-to-digital converters is operable to receive a bias voltage different than a bias voltage received by at least one other analog-to-digital converter and to convert the input signal into the digital output signal using the bias voltage. The method also includes generating a net digital output signal comprising a sum of the digital output signals.
Numerous technical advantages can be gained through various embodiments of the invention. Various embodiments of the invention may exhibit none, some, or all of the following advantages. For example, in one embodiment of the invention, a system is provided that uses a plurality of analog-to-digital converters to convert an input signal into digital output signals. In one embodiment, the analog-to-digital converters are Delta-Sigma converters, such as first-order or second-order converters. The lower-order Delta-Sigma converters typically include fewer components than converters previously used in other systems. This helps to reduce the cost of the communication system and the complexity of manufacturing the converters. Also, lower-order Delta-Sigma converters are typically more stable than other types of converters, and they are more insensitive to manufacturing tolerances. This also helps to reduce the complexity of manufacturing the converters.
Another technical advantage of some embodiments of the invention is that the presence of tones may be reduced or eliminated in the communication system. In one embodiment, the analog-to-digital converters use direct current (DC) bias voltages to generate the digital output signals. In a particular embodiment, each converter receives a different bias voltage, such as between +5 mV and xe2x88x925 mV. Biasing an analog-to-digital converter with a bias voltage may change the frequency at which tones are generated by that converter. In other words, the tones generated by one converter receiving a particular bias voltage may appear at higher or lower frequencies compared to the tones generated by another converter receiving a different bias voltage. By biasing multiple analog-to-digital converters with different bias voltages, the converters may generate tones that have different frequencies. Some of the converters may produce tones at the same frequency, but other converters produce tones at different frequencies.
Because a smaller percentage of the converters produce tones at the same frequency, a sum of the digital output signals produced by the converters may have fewer tones. For example, if the system includes one thousand analog-to-digital converters, one hundred of the converters may produce a tone at 500 MHz. However, nine hundred of the converters may not produce a tone at 500 MHz. Because only one tenth of the converters generate a tone at 500 MHz, the net power spectral density of the 500 MHz tone from all converters may fall below the net level of noise at 500 MHz from all converters. This helps to reduce or eliminate the tones produced in the communication system. This also helps to improve the signal-to-noise power ratio of the converters because less noise or undesired signals are being produced in the system.
A further advantage is that some embodiments of the invention may reduce or eliminate tones in the net digital output signal without reducing the effectiveness of or adding complexity to the converters. For example, some embodiments of the invention may reduce or eliminate tones in the net digital output signal without requiring the use of multiple feedback paths within the converters. Also, some embodiments of the invention do not require the introduction of noise to the input signal, which helps to prevent the decrease in the signal-to-noise power ratio associated with dithering the input signal.
Other technical advantages are readily apparent to one of skill in the art from the attached figures, description, and claims.